1. Field of the Invention
The present invention relates to a drive unit for a liquid ejection head for ejecting a liquid such as ink and the like by controlling a voltage applied to piezoelectric elements. In particular, the present invention relates to a drive unit which enhances the attenuation of meniscus vibrations. Further, the present invention also relates to a liquid ejection apparatus such as a printer which is equipped with such a drive unit, and to a drive method for a liquid ejection head.
2. Description of the Related Art
Ink-jet recording heads of an on-demand type comprise pressure chambers in which ink pressure is generated by piezoelectric elements or heat-generating elements, ink chambers supplying the ink into the pressure chambers, and nozzles ejecting the ink from the pressure chambers. Pressure is generated by applying drive signals to the elements corresponding to the printing signals, and ink droplets are ejected from the nozzles onto the recording medium. In particular, the advantage of the ink-jet recording heads using piezoelectric elements over the ink-jet recording heads of other types is that because no heat is used, the degradation of ink and clogging are prevented.
However, after the pressure was generated inside the pressure chamber and ink has been ejected, pressure fluctuations remain in the pressure chamber. As a result, the meniscus located inside the nozzle recovers to the nozzle tip, while vibrating at a resonance frequency inherent to the pressure chamber. However, it recently became necessary to drive the printing at a high frequency in order to conduct fine printing at a high speed. Because of such a necessity, if the next ejection is started while the residual fluctuations are not fully suppressed, the meniscus is not stabilized. For this reason, there is a spread in the amount of ejected ink and in the direction of droplet, which causes degradation of printing quality.
Japanese Patent Laid-open Publication No. 9-226106 discloses a process of applying, as shown in FIG. 6, a first signal, which causes the expansion of a pressure chamber, with a period shorter than the period Tc determined by the Helmholtz resonance frequency of the pressure chamber, and a second signal, which maintains the expanded state of the pressure chamber, at no more than xc2xd of Tc. As a result, the initial vibrations of the meniscus can be decreased and the meniscus vibrations can be rapidly attenuated.
Furthermore, Japanese Patent Laid-open Publication No. 10-24570 discloses a process of outputting, as shown in FIG. 7, a first signal causing the expansion of a pressure chamber, a second pulse causing the contract of the pressure chamber that assumed an expanded state and the ejection of an ink droplet from a nozzle, and a third pulse again expanding the pressure chamber. The vibrations of meniscus are attenuated by detecting the ambient temperature and controlling the initiation timing of the pulses.
Further, Japanese Patent Laid-open Publication No. 6-340075 discloses a process of temporarily terminating charging or discharging of piezoelectric elements by matching the timing and natural frequency of piezoelectric elements. As a result, the attenuated vibrations of piezoelectric elements are decreased.
Japanese Patent Laid-open Publication No. 9-226106 decreases the initial vibration of meniscus to shorten the attenuation period, however, the residual vibrations of meniscus cannot be effectively suppressed.
Furthermore, in Japanese Patent Laid-open Publication No. 10-24570, a control based on the results of detecting the ambient temperature is necessary to match the vibration timing of meniscus and the apparatus inevitably becomes complex.
Vibrations of meniscus have been thought to be mainly dependent on properties (viscosity and the like) of ink and the improvement of diaphragm properties has been assumed to make hardly any contribution to the modification of meniscus vibrations. However, if thin film piezoelectric elements are employed, the effect of the diaphragm properties (compliance and the like) on meniscus vibrations increases. When the compliance of diaphragm is large, the meniscus vibrations after ink ejection cannot be sufficiently attenuated. Conversely, when the compliance of diaphragm is small, the movement of diaphragm during ink ejection is small and ink cannot be sufficiently ejected.
Further, in the embodiment of Japanese Patent Laid-open Publication No. 6-340075, the thickness of piezoelectric elements is much larger than the thickness of diaphragm. Therefore, even if the vibrations of piezoelectric elements are controlled, the compliance of diaphragm is practically not changed. For this reason, a sufficient effect could not be demonstrated unless the timing was matched with the resonance frequency of piezoelectric elements, and the adjustment of timing was not simple.
Accordingly, it is an object of the present invention to provide a drive unit for a liquid ejection head, which makes it possible to suppress effectively the vibrations of meniscus with a simple configuration and device, and a liquid ejection apparatus using such a drive unit. It is another object of the present invention to provide a drive unit and a liquid ejection apparatus which make it possible to cause sufficient deflection of diaphragm when ink is ejected and rapidly attenuate the residual vibrations after ejection.
The present invention relates to a drive unit for a liquid ejection head in which contract of a pressure chamber and ejection of the liquid are caused by the application of a voltage to a piezoelectric body, to a drive method, and to a liquid ejection apparatus equipped with such a drive unit and liquid ejection head. The voltage applied to a piezoelectric thin film has an electric potential stable period in the course of returning from an electric potential when pressure chamber has contracted for the ejection of liquid to another electric potential prior to a drive for the next ejection. Providing the electric potential stable period suppresses deformation of piezoelectric thin film and vibrations of diaphragm. As a result, the compliance of piezoelectric thin film and diaphragm can be reduced and the natural vibration period Tc of pressure chamber can be decreased. Therefore, the meniscus vibrations can be effectively suppressed with a simple configuration and device.
Furthermore, in accordance with the present invention, a circuit connected to the piezoelectric thin film is disconnected within a specific period during damped meniscus vibrations after liquid ejection. Disconnecting the circuit connected to the piezoelectric thin film makes it possible to suppress reliably the electric potential changes on the piezoelectric thin film. Moreover, it is preferred that in the aforesaid drive unit, the disconnection of the circuit be intermittently conducted during damped meniscus vibrations.
Further, in accordance with the present invention, during damped meniscus vibrations after the liquid ejection, the natural period Tc of the pressure chamber is made shorter than the natural period during the liquid ejection. As a result, the natural period during damped meniscus vibrations after the liquid ejection can be shortened with respect to the natural period during liquid ejection. Then, the diaphragm can be sufficiently deflected during liquid ejection and the residual vibrations can be rapidly attenuated after the ejection.
An especially significant effect can be obtained when the polarization generated in a piezoelectric thin film during voltage application is no less than 0.25 C/m2.
It is also preferred that the thickness of the piezoelectric thin film be 0.5 xcexcm through 2 xcexcm. A small thickness of the piezoelectric thin film can greatly change the compliance of the piezoelectric thin film and the diaphragm and can effectively suppress the residual vibrations. Furthermore, it is preferred that the time period from the pressure chamber contract to the beginning of the electric potential stable period be shorter than the natural period of the pressure chamber. As a result, the residual vibrations can be rapidly attenuated.